Convergent method via α-cyano ethers: A powerful strategy for synthesizing ladder-shaped polyethers

Ladder-shaped polyether (LSP) toxins attract considerable attention among synthetic organic chemists due to their potent biological activities coupled with their unique molecular structures. Although a number of methods for synthesizing LSPs have been reported, developments of convergent and practical methods are necessary for efficient synthesis of LSPs in order to reveal their biological function at the molecular level and because of their limited availability from natural sources. During the course of our synthetic studies of LSPs, we developed a convergent method via α-cyano ethers (α-cyano ether method). An additional two rings can be constructed through the coupling of fragments, which is an advantage of this strategy for the construction of polycyclic systems. Medium-sized cyclic ethers are formed by ring-closing metathesis, and tetrahydropyran rings are stereoselectively constructed by reductive etherification or thioacetal formation followed by alkylation. The α-cyano ether method was successfully applied to the synthesis of the partial structure corresponding to the ABCDEFGHIJ ring system of yessotoxin, where utilization of a microflow reactor was found to be effective for reductive etherification. The α-cyano ether method was also applicable for synthesizing the WXYZA-B-C-ring system of maitotoxin, and designed artificial tetra-, hepta-, and decacyclic LSPs.